i) Field of the Invention
This invention relates to the recovery of rhodium from aqueous solution, more especially to the recovery of rhodium from industrially produced aqueous precious metal solutions.
ii) Brief Description of Prior Art
The separation and purification of rhodium (Rh) from the other precious metals is one of the most difficult and pressing areas in precious metal refining at the present time. This situation arises mainly because of the complex solution chemistry of Rh in chloride containing aqueous solutions. The complexes formed by Rh in these types of solutions are such that modern recovery processes such as solvent extraction (SX) which have been implemented for the recovery of the other platinum group metals (PGMs) cannot easily be applied to the recovery of Rh and thus far, no industrially acceptable solvent extraction system has been developed for Rh.
Rhodium occurs together in nature with the other PGMs as well as with silver and gold either as native alloys in placer deposits or in lode deposits or in lode deposits where they are associated with Cu and Ni. It is from the lode deposits that the PGMs are most frequently recovered. Because the PGMs occur together, it is important to devise techniques to separate them and to purify and recover each of the metals separately. Originally PGMs were separated after dissolution in oxidizing chloride leach liquors by the application of a series of precipitation-dissolution steps adopted from analytical chemistry methods. This was the most common route until about the middle nineteen seventies. Since then, the major refining companies have considerably modernized their processes by implementing the more efficient separation technique of solvent extraction and to a lesser degree, ion exchange.
In virtually all precious metal recovery systems, Rh is the last metal recovered and it is recovered through a complicated precipitation technique rather than through the more modern and efficient technique of solvent extraction.
The precipitation-dissolution scheme for the recovery of Rh is not considered satisfactory by most PGM refiners because of its numerous drawbacks. It is a lengthy process, sometimes taking as long as 4 to 6 months for the final recovery of pure Rh metal and therefore, there is a high value of metal which is locked-up in the processing plant. The technique is also quite tedious as the precipitation must be carried out a number of times in order to ensure that the final product is of acceptable purity and this makes the overall process labour intensive and costly.
In the precipitation - purification method, the first step involves the formation of the nitrite complex [Rh(NO.sub.2).sub.6 ].sup.3- from RhCl.sub.6.sup.3-. Because this complex is extremely stable to hydrolysis, the impure Rh-containing solution can be subjected to neutralization with NaOH in order that some of the impurities be precipitated through hydrolysis. After a filtration stage, the Rh in solution is precipitated with ammonia and sodium (from the NaOH) as Na(NH.sub.4).sub.2 [Rh(NO.sub.2).sub.6 ] which is a partially selective precipitation step over the other PGMs which may also be present :n the Rh solution. For this precipitation, however, it is important that a high concentration of ammonia be used in order to suppress the solubility of this Rh complex to achieve almost complete Rh precipitation. After another filtration stage, the precipitate is redissolved in HCl and depending on the purity of the solution, the process is started over at the nitriting step.
It is this cycle of precipitation-dissolution stages that renders this process inefficient and tedious. Once the ammonia-nitrite Rh complex is of acceptable purity, the final dissolution in HCl is followed by the precipitation of Rh with ammonia to give (NH.sub.4).sub.3 [RhCl.sub.6 ]. It is not only important that the concentration of ammonia be high to suppress the solubility of the Rh compound but, as well, that the chloride concentration be high since it is the hexachloro-complex which is precipitated and therefore, the hexachloro-complex must be available in solution. The last step involves the reduction of Rh to the metallic state either directly from this solution with formic acid or by calcining the complex in the presence of H.sub.2 (g) at about 1000.degree. C.
Rh metal is of high value (about $4,000 U.S./oz in mid-1991) and with the rapidly increasing demand for automobile catalytic converters which utilize Rh, the need to develop more efficient recovery processes such as solvent extraction for Rh is becoming more urgent. The difficulty in developing such systems, however, lies in the chemical complexity of Rh in Cl-containing aqueous solutions.
The main oxidation state of Rh is +III although +I and others are known to exist though to a much lesser extent. The anionic complexes of rhodium are more labile than those of other PGMs, whereas the cationic and neutral complexes are quite inert.
Rhodium (III) readily forms octahedral complexes, as do most d.sup.6 configurations, with anions, halides, and oxygen-containing ligands. In terms of solvent extraction, highly charged octahedral complexes such as RhCl.sub.6.sup.3- are particularly difficult to extract due to steric effects because it is difficult to pack three organic molecules around a single anion.